Brush them with either a beaten egg, w/ salt , or cream, w/ salt. The Salt is what browns them. When you want a browner crust on anything, casserole or baked good, RAISING the heat will do it, not lowering the heat.

I have never used salt to brown a biscuit just for a little extra flavoring. Browning agents are either the egg wash as I mentioned above OR brush tops with a little melted butter. Do not extend the baking time or temperature as this will effect the quality of the biscuits texture.